In recent years, with an increase in communication demands, for example, it has been demanded to increase the number of optical fiber cores, optical signal capacity per wavelength, and the number of wavelength division multiplexing (WDM) channels, thereby enhancing the transmission capacity. However, because of high laying costs of optical fibers, it has been required to increase mainly the optical signal capacity and the number of WDM channels without increasing the number of optical fiber cores, to enhance the transmission capacity. The transmission device achieves communication using light wavelengths in a conventional band (C band) having a range of 1530 nm to 1565 nm, for example. However, using only the C band, there is a limit to enhance the transmission capacity.
Examples of the optical communication band include the band in the range of 1260 nm to 1675 nm, including the C band, the 0 band, the E band, the S band, the L band, and the U band. The 0 band (Original Band) is the band in the range of 1260 nm to 1360 nm, the E band (Extended Band) is the band in the range of 1360 nm to 1460 nm, and the S band (Short Band) is the band in the range of 1460 nm to 1530 nm. The L band (Long Band) is the band in the range of 1565 nm to 1625 nm, and the U band (Ultra-Long Band) is the band in the range of 1625 nm to 1675 nm.
Thus, attempts have been made to use the communication bands such as the L band and the S band in addition to the C band, thereby further enhancing the transmission capacity in the transmission device. The multi-band WDM system using a plurality of wavelength bands requires a wavelength converter or the like for wavelength-converting, for example, multiplexed light in the C band into multiplexed light in the L band. The wavelength converter makes use of nonlinear optical phenomenon of the four-wave mixing (FWM), for example. FIG. 9 is a view illustrating an example of a conventional wavelength converter 100. The wavelength converter 100 illustrated in FIG. 9 has an input port 101, a pump light source 102, a WDM coupler 103, an optical circulator 104, and a polarizing beam splitter (PBS) 105. The wavelength converter 100 has a polarization-maintaining nonlinear fiber 106, an optical BPF 107, and an output port 108. For example, in the case of the wavelength converter 100 that converts multiplexed light in the C band into multiplexed light in the L band, the multiplexed light in the C band before wavelength conversion is signal light, and the multiplexed light in the L band after wavelength conversion is converted light.
The input port 101 is a port that inputs the signal light. The pump light source 102 is a light source that outputs pump light. The WDM coupler 103 multiplexes the signal light from the input port 101 and the pump light from the pump light source 102. The optical circulator 104 outputs the signal light and the pump light from the WDM coupler 103 to the PBS 105, and outputs the converted light after wavelength conversion, the signal light, and the pump light from the PBS 105 to the optical BPF 107. The PBS 105 divides the signal light and the pump light before wavelength conversion into vertically-polarized signal light and pump light and horizontally-polarized signal light and pump light. The PBS 105 inputs the vertically-polarized signal light and pump light in an X direction of the polarization-maintaining nonlinear fiber 106, and inputs the horizontally-polarized signal light and pump light in a Y direction of the polarization-maintaining nonlinear fiber 106.
Using four-wave mixing of the horizontally-polarized pump light and the horizontally-polarized signal light, the polarization-maintaining nonlinear fiber 106 wavelength-converts the horizontally-polarized signal light into the horizontally-polarized converted light while maintaining the horizontally-polarized signal light. Then, the polarization-maintaining nonlinear fiber 106 outputs the horizontally-polarized converted light, signal light, and pump light to the PBS 105. Using four-wave mixing of the vertically-polarized pump light and the vertically-polarized signal light, the polarization-maintaining nonlinear fiber 106 wavelength-converts the vertically-polarized signal light into the vertically-polarized converted light while maintaining the vertically-polarized signal light. Then, the polarization-maintaining nonlinear fiber 106 outputs the vertically-polarized converted light, signal light, and pump light to the PBS 105.
The PBS 105 multiplexes the vertically-polarized converted light, pump light, and signal light and the horizontally-polarized converted light, pump light, and signal light, and outputs the converted light after wavelength conversion, signal light, and pump light to the optical circulator 104. The optical circulator 104 outputs the converted light after wavelength conversion, signal light, and pump light to the optical BPF 107. The optical BPF 107 extracts only the converted light from the converted light after wavelength conversion, signal light, and pump light, and outputs the extracted converted light to the output port 108. That is, the wavelength converter 100 may wavelength-convert the signal light in the C band into the signal light (converted light) in the L band.
However, to improve the wavelength conversion efficiency of the four-wave mixing, the wavelength converter 100 has to match the polarization state of the signal light with the polarization state of the pump light. The normal optical fiber has two polarization modes of a horizontal polarization mode and a vertical polarization mode, which are orthogonal to each other. In the normal optical fiber, the core becomes noncircular due to a stress exerted on the optical fiber and thus, the polarization of light during propagation varies, such that the polarization state of the signal light does not become fixed in an input stage of the wavelength converter 100. Further, to match the polarization state of the signal light with the polarization state of the pump light, the pump light inputted to the wavelength converter 100 also has to have two orthogonal polarization states.
FIG. 10 is a view illustrating an example of the polarization state of the pump light in a polarization-maintaining nonlinear fiber 120. In the polarization-maintaining nonlinear fiber 120 illustrated in FIG. 10, a circular stress application part 122 is disposed on each side of a core 121. For example, the stress application part 122 has different refractive indexes in the horizontal polarization mode and the vertical polarization mode and thus, has different propagation rates. As a result, when a polarized wave of the pump light is inputted at 45 degrees, the polarization state varies in the signal propagation direction of the fiber to generate the vertically-polarized pump light and the horizontally-polarized pump light, which are orthogonal to each other.
For example, related arts are disclosed in Japanese Laid-open Patent Publication Nos. 2000-75330, 2005-12358, and 2000-258811.
In the wavelength converter 100, to ensure the two orthogonal polarization states of single pump light, the pump light has to be inputted to the PBS 105 in the polarization state of 45 degrees. Thus, in the wavelength converter 100, to maintain the pump light in the polarization state of 45 degrees, it is required for connecting from the pump light source 102 to the polarization-maintaining nonlinear fiber 106 via a polarization-maintaining fiber. Further, in the section from the pump light source 102 to the polarization-maintaining nonlinear fiber 106, the WDM coupler 103, the optical circulator 104, and the PBS 105 each use a polarization-maintaining fiber-type optical component. As a result, the polarization state of the pump light in the input stage of the PBS 105 may be maintained to 45 degrees, thereby inputting and vertically-polarized pump light and horizontally-polarized pump light to the polarization-maintaining nonlinear fiber 106. That is, to maintain the polarization state of the pump light of 45 degrees in the section from the pump light source 102 to the PBS 105, polarization-maintaining fiber-type optical components are used in the section, increasing cost of components of the wavelength converter 100.
Moreover, since an output of a semiconductor laser diode (LD) used as a current pump light source is about 18 dBm, pump light having about 20 dBm has to be inputted to the polarization-maintaining nonlinear fiber used for wavelength conversion of the wavelength converter. Accordingly, an optical amplifier for amplifying pump light is required in an output stage of the pump light source. However, optical components such as an optical isolator, an erbium doped fiber (EDF), and an optical coupler in the optical amplifier also are polarization-maintaining fiber-type optical components. Such polarization-maintaining fiber-type optical components have high scarcity value and high component cost. In other words, the component cost of the wavelength converter 100 is high.
In consideration of the above-mentioned situation, it is desirable to provide a wavelength converter capable of inputting pump light in orthogonal polarization states to a nonlinear medium while decreasing the number of polarization-maintaining fiber-type optical components.